GB817066A - Improvements in or relating to the manufacture of dispenser cathodes - Google Patents

Abstract

817,066. Cathode materials and processing. PHILIPS ELECTRICAL INDUSTRIES Ltd. July 19, 1957 [July 24, 1956], No. 22936/57. Class 39(1). In a method of making impregnated dispenser cathodes, the porous cathode body to be treated is embedded in freshly alkaliearth oxide and then heated rapidly, for example to about 2000‹C., in an inert atmosphere such as a mixture of hydrogen and nitrogen or a rare gas at a pressure of at least one half an atmosphere so that the oxide melts and impregnates the cathode body. In a modified method, the porous body is embedded in an alkali earth compound or mixture of such compounds such as the carbonates, peroxides, hydroxides or nitrates which decompose into oxide before a significant amount of the original material impregnates the body. Particular care is taken rapidly to transverse the temperature range at which the oxy-carbonate phase exists. The impregnating materials used are BaO alone or mixed with other alkali earth oxides, with carbonates preferably used as starting materials in the modified method described above, a preferred mixture being BaCO 3 and SrCO 3 in proportions lying between 1 : 2 and 5 : 1. In all cases it is preferred that the final BaO content should be at least 25 per cent. The cathode body to be impregnated consists of pure tungsten, a tungsten-molybdenum alloy or a tantalum-molybdenum alloy. In the case of tungsten cathodes the density of the body is between 55 per cent. and 90 per cent. of that of the solid metal and the sintering temperature is preferably above 2100‹C. After cooling the cathode body, the adhering layer of oxide is removed and the cathode is mounted in an evacuated electron tube and activated by heating for some time at 1250‹C. Impregnation times vary from 15 seconds to 10 minutes. The life times of various cathodes are given.